I have two layers of polygons with landcover classes(meadows, cropland, forest, ect) They overlap in both directions, i.e like a zipper. I need to dissolve them by one attribute (Class) so I can have the total area of each polygon... but in the overlapping sections sometimes the landcover classes don´t match.

If I try UNION, the algorithm creates new features for this overlapping regions, but also creates another AREA column and in some cases I have values for both columns simultaneously (I have now 2 AREA columns: one for each layer I "unioned").

I also tried the merge layers of the MMQGIS plugin, but the attributes are not summed up. And dissolving by class just creates an empty layer.

(The image is the union of the two layers. The arrows indicate which layer is overlapping over the other. Both images are corrected in terms of projection)

Does someone knows how to do something like this?
I work on QGIS 2.12.3 on a PC.

If you use the "Difference" tool (before or after Dissolve), you won't have overlapping polygons anymore. Not sure if that's what you're trying to achieve though. Also, you may be able to compute the area with an expression instead of using your "AREA" attribute, depending on your projection.
– ArMoraerFeb 3 '16 at 20:24

@ArMoraer I tried the difference tool before dissolving but I had to do it Layer1-Layer2 AND Layer2 - Layer1. Because the layers overlap like a zipper (each one on top of each other). And when I did this there is an empty space between my layers and I can not dissolve them anymore.
– delVientoFeb 4 '16 at 7:48